<!DOCTYPE html>
<html>
	<head>
		<meta charset="UTF-8">
		<title></title>
	</head>
	<body>
		<script type="text/javascript">
			alert(a)
			alert(b)
			var a=10;
			var b =20;
			//变量的提升，在js运行时会把变量提升顶部，默认给undefined
		</script>
	</body>
</html>
